We have focused on a one-dimensional metal-dimer system with dynamical spin-charge-lattice coupling, exhibiting many kinds of charge-ordered states associated with metal-insulator transitions. In this report, we present on an MMX-chain system in which there exists 1-D 【triple bond】-I-M-M-I-【triple bond】 chains. Some of the possible 1-D structural modes for the MMX chain are shown below.(1)【triple bond】M^<2.5+>-M^<2.5+>-X-M^<2.5+>-M^<2.5+>-X-M^<2.5+>-M^<2.5+>-X【triple bond】(2)【triple bond】M^<2+>-M^<2->-X-M^<3+>-M^<3+>-X-M^<2+>-M^<2+>-X-M^<3+>-M^<3+>-X【triple bond】(3)【triple bond】M^<2+>-M^<3+>-X-M^<2+>-M^<3+>-X-M^<2+>-M^<3+>-X-M^<2+>-M^<3+>-X【triple bond】(4)【triple bond】M^<2+>-M^<3+>-X-M^<3+>-M^<2+>-X-M^<2+>-M^<3+>-X-M^<3+>-M^<2+>-X【triple bond】According to our recent work, the Ni_2(MeCS_2)_4I salt was found to exhibit a field-induced phase transition accompanying a spin transition between a 1-D Heisenberg AF magnetic spin system with J 〜 -50 K (a-phase) and Curie spin system with J 〜 0
… MoreK (b-phase). In order to investigate two different spin state, several physical measurements were carried out on Ni_2(MeCS_2)_4I.It exhibits a semiconducting behavior in the temperature range of 180 - 370 K, which is consistent with a Mott-Hubbard insulator. The XPS spectra shows a trapped-valence state of +II and +III. Each phase is considered to have the 1-D structural mode of -I-Ni^<2+>-Ni^<3+>-I-Ni^<3+>-Ni^<2+>-I-Ni^<2.5+>-Ni^<2.5+>-I-, which was confirmed by Raman, IR, UV-VIS-NIR spectra and magnetic susceptibility.At 2 K, if the magnetic field of 5 T is applied, the a-phase is transformed to the b-phase, which can be obtained as a metastable state even at room temperature and zero-field. On the other hand, if the magnetic field of 5 T is applied at 300 K, the b-phase is reversibly transformed to the a-phase. This field-induced phenomenon is considered to be derived from an orbital degree of freedom, resulting the field-induced orbital ordering. Detailed investigations are in progress. Less